Chapter 48 classifies paper and paperboard by kind, by grammage in g/m² and by surface treatment. Those numeric thresholds do the work: the same paper changes heading depending on whether it is coated, impregnated or merely calendered.
The sharpest boundary is with Chapter 49. Printed paper stays here as long as the printing is incidental — printed wrapping paper, a ruled register. Once the text or image becomes the point of the article itself, it belongs to Chapter 49.
Every number opens on the lines beneath it, down to the ten-digit classification number.